A METHOD OF ENCAPSULATING COMPONENTS ON A PRINTED CIRCUIT

Abstract: Encapsulant material (30) is applied to a component on a printed circuit (12, 13) by means of a stencil, conveniently in a screen printing process, to enable a known and repeatable quantity of encapsulant material (30) to be deposited in order to minimize the thickness of the overall device. As well as the amount of material deposited a gasket (16) on the substrate (12) of the printed circuit can be used to control the spread of encapsulant on compression by a capping member for the encapsulant prior to curing.

Description: A METHOD OF ENCAPSULATING COMPONENTS ON A PRINTED CIRCUIT This invention relates to a method of encapsulating components on a printed circuit and is particularly applicable to the fabrication of devices where the overall depth of the device is restricted, for example in the manufacture of what are commonly referred to as "plastic cards", that is such cards which are carried by individuals and used for such purposes as recording or authorising transactions and/or for authorising entry to buildings or parts thereof. (Note that the term "printed circuit" as used in this specification should be considered to refer to any system of conductive tracks on an insulating substrate, whether such tracks are formed by printing, etching, vapour deposition or any other technique.) A previous proposal for fabricating plastic cards is as schematically illustrated in Figure 1. A printed circuit board comprising conductive tracks 6 has components 2 attached thereto, which components are potted in encapsulant 3. The encapsulant 3 is applied in such a quantity as to ensure that the components 2 are totally encapsulated. However the thickness of this encapsulant 3 determines the thickness that the protective layers 4 and 5, typically of pvc material, have to be in order to accommodate the encapsulant. Furthermore it is desirable to place the substrate 1 in the centre of the card, i.e. layers 4 and 5 have to be of equal thickness, in order to reduce strain on the printed circuit due to flexing which a thin plastic card is inevitably subject to. The present invention arose as a consequence of designing a card comprising such potted components which card was not to exceed the dimensions of a conventional plastic card. According to the present invention there is provided a method of encapsulating components on a printed circuit in which the encapsulating material is applied using a stencil. By using a stencil it is possible to accurately determine the amount of encapsulant deposited and thereby apply repeatedly a fixed volume of encapsulant enabling the volume deposited to be minimized. The use of a stencil is also particularly advantageous if a plurality of circuits are on a single substrate, for then a corresponding number of apertures in a stencil enables a plurality of components to be encapsulated at the same time. It is particularly advantageous if the encapsulating material is screen printed, the stencil comprising the mask of the screen printing process. Preferably the printed circuit comprises a substrate layer with a number of tracks thereon, the substrate comprising a raised portion which forms a gasket for the encapsulant material. The term "gasket" in the context of this specification means a device which prevents the spread of a material and does not require that the gasket forms a seal between two members. The gasket enables the spread of the encapsulant to be restricted, thereby confining the predetermined amount of material deposited to a predetermined area further enabling the quantity of encapsulant deposited to be minimized by retaining it in the desired position. Advantageously the gasket is a dielectric material for this will not short any conductive tracks over which it passes and can also be easily applied to the printed circuit by standard techniques. Advantageously the gasket defines an area to which it is desired to restrict the encapsulant, the method comprising depositing encapsulant within the area and subsequently compressing the encapsulant between the substrate and a capping layer until a meniscus of the encapsulant material comes into contact with the gasket. Preferably the capping layer comprises a further gasket of similar shape to the first, to which gasket a second meniscus of the encapsulant extends. This thereby defines the two facing surface areas of the encapsulant material and preferably the encapsulant is compressed until the capping layer is restrained by a spacer between the two gaskets. It will be appreciated that this enables a predetermined amount of material to extend to predetermined shapes top and bottom and adopt a preset thickness determined by the thickness of the spacer, therefore all parameters are known and the amount of encapsulant required can be minimized such that it is just sufficient to pot the components. This method is particularly advantageous for fabricating a plastic card wherein the thickness of the card is of importance, for dimensions in excess of the standard plastic card could prejudice the commercial success of such a card. One embodiment of the invention will now be described, by way of example only, with reference to Figures 2 to 11 of the accompanying drawings in which like numerals have been used to indicate like parts, and of which: Figure 1 schematically illustrates a previously proposed arrangement (not in accordance with the invention) of electrical components in a plastic card; Figure 2 is a perspective view of a plastic card in accordance with the present invention; Figure 3 is a plan view of the card of Figure 2 having its top sheet 4 removed to reveal the printed circuit; Figure 4 is a cross-section through the card along the line I - 1 of Figure 3; and Figures 5 to 11 illustrate various stages in the production of the plastic card depicted in Figures 2 to 4. Referring first to Figure 2 there is shown a perspective view of the final card which has the same external dimensions as a standard "plastic card". The card contains an integrated circuit. On Figure 2 it is possible to see edge portions of a substrate of the integrated circuit, exposed at a central part of each edge of the card. The integrated circuit communicates with interrogation units via an inductive link located at appropriate locations. The integrated circuit would normally contain a memory device and could be used for any number of purposes, for example recording banking transactions or recording zones of buildings etc to which entry has been gained by use of the card as an identity card. Referring to Figure 3 there is illustrated a plan view through a section of the card 10 of Figure 2 in the plane of the card. From this and the cross-section along line I - 1 illustrated in Figure 4 it can be seen that a printed circuit 11 comprises epoxy/glass substrate 12 and conductive tracks 13, a substantial portion of which form conductive loop 14. Shaded regions 15 and 16 comprise of a thermoset dielectric material. The purpose of the region 15 is to insulate a silver conductor 17 from the inductive coil 14. The purpose of dielectric layer 16 will be explained later. An integrated circuit and capacitive components, not shown in Figures 3 or 4, are contained within a capsule-like element 19 which is separated by cut 18 from the rest of the substrate 12. The region 20 of the substrate 12 is lowered below the plane of the printed circuit 11, the integrated circuit and capacitive components being located in potting compound 21 sandwiched between the portion of the substrate 20 and a capping portion 22 of the same material as the substrate 12. The printed circuit 11 and element 19 are sandwiched between two outer sheets 23 and 24 of PVC thermoplastics material and two intervening layers (not shown in Figures 3 or 4), of polyester which is coated on both sides with a thermally activated catalyst adhesive by which the laminated structure is adhered. This polyester acts as a reinforcing layer preventing element 19 "breaking out" of the PVC layers 23 and 24. The fabrication process of the card illustrated in Figures 2, 3 and 4 begins with a substrate sheet 12 of copper-clad epoxy/glass which is etched to form a large number of identical printed circuits 13, each as illustrated in Figure 5. On top of each printed circuit is printed a thermoset dielectric material indicated by the shaded regions 15, 16 which is cured in place. The function of circular part 16 is explained below. The linear part 15 serves as an insulator for the printed conductive link 17 for the inner end of a coil 14 defined by part of the printed circuit 13. Separated from a main part of the substrate by lines of weakness not shown are a number of strips (not shown), each carrying printed patterns 25 (only one of which is illustrated), with apertures 26 therein, the patterns 25 defining areas which ultimately become the top reinforcing caps 22 of the elements 19. The substrate carrying the etched patterns is placed on a bed of a screen printing machine (not shown) and a screen placed over it. A squeegee is then used to print a low ionic epoxy encapsulant adhesive material onto positions 27 as shown in Figure 6. This is a mixture of a resin and a catalyst which sets hard when cured. Suitable materials are, for example, available from Ablestick, Encaremix, or Dexter Hisol. The substrate is then placed in a "pick-and-place" machine which places components comprising of capacitors 28 and silicon chips 29, shown in Figure 7, onto the epoxy which acts as an adhesive to hold them in place. The silicon chips 29 at this stage are "naked", that is to say they are not encapsulated. A notable feature of this process is that the epoxy is applied to areas where there is no copper layer, this being unnecessary because of the adhesive attachment of the components. A saving of 35 microns in thickness is thus achieved as compared with arrangements where components are soldered on top of a copper track. It will be appreciated that this reduction of thickness may be of crucial importance in situations where there may typically be a requirement for the entire assembly not to exceed 760 microns. An advantage of using epoxy adhesive is that if suitably selected it remains in its adhesive state for a sufficient time period which exceeds the maximum period during which the screen printing machine is not being operated. This avoids the need to clean down the equipment. The sheet substrate carrying the etched patterns and respective components positioned on it, is then baked until the epoxy has gelled, i.e. set but not hardened. This takes place under a flow of nitrogen to prevent oxidation of the copper. The sheet is then placed on the work- holder of a wire bonding machine where it is held in position by a vacuum. Suitable machines for this purpose are commercially available. Wire connections are then made between contacts on the individual components to appropriate parts of the printed copper circuitry. This is done by an ultrasonically assisted diffusion welding process. The sheet is then placed back in the screen printer with a different stencil in place. This stencil is much thicker, its thickness being selected so that the same epoxy encapsulant/adhesive now to be deposited over the components is sufficient to cover them completely. Notably, this material is the same as that which was used for the adhesive. It does not have to be the same but it preferably has similar physical characteristics. After the removal of the stencil, the sheet is as shown in Figure 8, the components being encapsulated by the encapsulant 30. Figure 9 shows in cross-section the next stage of the process where a copper spacer 31 having a plurality of apertures 32 (corresponding to each of the regions on the sheet having encapsulant 30 deposited thereon) is located on the sheet, 12. Previously placed on the copper sheet is each of the now separated strips 33, previously referred to, to form regions defined by printed patterns 25, from which regions reinforcing caps 22 will be formed. The spacer 31, with strips 33 located on it by means of pins (not shown for clarity), has been placed on top of the substrate. The whole arrangement is then pressed such that the patterns 25 are pressed into contact with the spacer 31 which is thus pressed closely down onto the circular part 16 of the dielectric material. It also presses the portions of the strip 33 defined by the patterns 25 onto the, still soft, epoxy encapsulant/adhesive thereby pulling the entire assembly down to the desired height. During this process the encapsulant spreads out as shown in detail in Figure 10, but not as far as the edges of the spacer sheet. It is prevented from doing so by its meniscus acting against the inner edges of the copper pattern 25 and dielectric ring 16, which meniscus thereby defines the radius of the encapsulant. The whole assembly is now placed in an oven and cured at a temperature of 150°C. This fully gels the encapsulant/adhesive both under the components and the encapsulant portion. The assembly is now placed on a rule die which forms cuts 18 which can be seen in Figure 3. These cuts are "horseshoe-shaped" and configured so that their free ends correspond with the slots 26 (see Figure 5) in the strip 33. Note at this stage that the ends of each cut are located on the copper pads 35 of Figure 3. The cutter presses through the structure as illustrated by dotted lines 36 in Figure 10, leaving the element 19 on a limb of the substrate 11 , as is best seen from Figure 3, and leaving the spacer 31 and remaining portions of the strips 33 free to be removed. It will be noted from Figure 3 that the electrical connections to the element run parallel to an edge of die card, in which direction the card is most resistant to bending, as opposed to across the hinge line which runs across the corner of the card where it is most susceptible to bending. Using another rule die, cruciform shapes are cut out of the assembly to give each printed circuit the shape illustrated in Figure 3. This removes the epoxy/glass substrate from those areas which are to become the comers of the finished cards. It is notably these comer parts which are most subject to the type of manipulation which encourages de-lamination. The printed circuit 1 with reinforced element 19 is now placed, as shown in Figure 11, between two outer sheets 37 and 38 of thermo plastics material in the pvc family with the inter¬ position of polyester layers coated on both sides with a thermally activated catalyst adhesive. The assembled sandwich is placed in a press where it is heated to cause lamination. During this stage the elements 19 imbed themselves in each of the sheets of thermo-plastic material in such a way as to tend to centralise themselves between opposite faces leaving the plane of the substrate sheet 1 on the central axis as shown in Figure 4. The press now opens and the assembly is removed to a cutting machine where the individual cards as illustrated in Figures 2 and 3 are cut out. Although in the specific embodiment illustrated each electronic element is formed integrally with the printed circuit, each element could alternatively be formed on a separate part of the printed circuit which is subsequently connected to the main part of the printed circuit, the element being retained in an aperture in the main printed circuit formed by a circular or similar cutter by being "snapped" into the aperture, the end faces of the element being of slightly greater diameter than the aperture. The elements would be formed by a process very similar to that disclosed except they would be formed on a substrate having a far greater density of elements, from which substrate they would eventually be cut. The elements can be cut from the substrate having a limb extending from the element by which they are connected to the main portion of the printed circuit of a card by normal soldering of conductive tracks on the limb to tracks on the printed circuit or by similar techniques. The card shown in the illustration shows an electronic element connected to an inductive loop 14 in a contacless card. The electronic element could alternatively be connected by means of a limb to an electrical contact or other component of a card.

Claims: CLAIMS 1. A method of encapsulating components on a printed circuit in which encapsulating material is applied using a stencil. 2. A method as claimed in claim 1 wherein the encapsulating material is screen printed, the stencil comprising the mask of the screen printing process. 3. A method as claimed in claim 1 or 2 wherein the printed circuit comprises a substrate layer with a number of tracks thereon, the substrate comprising a raised portion which forms a gasket for the encapsulant material. 4. A method as claimed in any preceding claim wherein the gasket is a dielectric material. 5. A method as claimed in claim 3 or 4 wherein the gasket defines an area to which it is desired to restrict the encapsulant, the method comprising depositing encapsulant within the area and subsequently compressing the encapsulant between the substrate and a capping layer until a meniscus of the encapsulant material comes into contact with the gasket. 6. A method as claimed in claim 5 wherein the capping layer comprises a further gasket of similar shape to the first, to which further gasket a second meniscus of the encapsulant extends. 7. A method as claimed in claim 5 or 6 wherein the encapsulant is compressed until the capping layer is restrained by a spacer between the two gaskets. 8. A method as claimed in any preceding claim wherein the printed circuit and encapsulated component are embedded in protective material. 9. A method as claimed in any preceding claim for fabricating a plastic card as hereinbefore defined. 10. A method of fabricating a card substantially as hereinbefore described with reference to Figures 2, 3, 4, 8, 9 and 10 of the accompanying drawings. 11. A card fabricated by the method of any one of claims 1 to 10.

Inventors: JARVIS CHARLES RICHARD, JARVIS, CHARLES, RICHARD
Applicants: GEC AVERY LTD, JARVIS CHARLES RICHARD, GEC AVERY LIMITED, JARVIS, CHARLES, RICHARD
Application year: 1994
Main classification: G06K19
IPC codes: B42D15, G06K19, H05K3